Various kinds of thermal transfer recording methods are well-known. Of them, a thermal fusion transfer recording method and a thermal sublimation transfer recording method are known as widely-used methods.
The thermal fusion transfer recording method is such an image forming method that, by applying energy corresponding to image information to a heating device such as thermal head, heat is applied to a thermal transfer sheet in which a thermofusible color layer is disposed on a substrate, the thermofusible color layer containing a thermofusible binder (e.g., resin or wax) and color materials (e.g., pigments) dispersed in the binder, to thermally transfer the color materials and the binder to a transfer receiving medium such as paper, thereby forming an image.
The thermal sublimation transfer recording method is such an image forming method that, by use of a thermal transfer sheet in which a sublimation color layer containing a sublimation dye is disposed on a substrate, the sublimation dye is thermally transferred to the dye receiving layer of a thermal transfer image receiving sheet, thereby forming an image. According to this method, a full color image can be reproduced by transferring three or more colors to the dye receiving layer of the thermal transfer image receiving sheet for gradation image printing, with controlling the amount of heat applied with a thermal head when thermal transfer.
In these thermal transfer sheets, to prevent fusion between the substrate sheet (the rear side (not the color layer side) of the substrate sheet) and the thermal head (heating means), a heat resistant slipping layer is disposed on the opposite side from the color layer side of the substrate sheet, or a primer layer is further disposed between the substrate sheet and the heat resistant slipping layer.
However, with the speed up of thermal transfer printers in recent years, there is a tendency for thermal energy generated from thermal heads to increase, thus leading to problems such as fusion sticking of the thermal head and the heat resistant slipping layer of the thermal transfer sheet, wrinkles in printed images, and rupture in the thermal transfer sheet.
A thermal transfer sheet is disclosed in Patent Document 1, which has flexibility and heat-resistance and thus is resistant to rupture. In the thermal transfer sheet, a specific heat resistant slipping layer is formed on the opposite side from a color layer side of a substrate sheet, via a specific primer layer composed of a crosslinking agent and a specific binder resin. In Patent Document 1, it is described that from the viewpoint of flexibility, etc., the crosslinking agent is preferably a titanium chelate agent or an isocyanate compound, and the binder resin is preferably a polyvinyl alcohol resin or a polyvinyl butyral resin.